Materials suffer damage over time. For example, an airplane fuselage is typically composed of a composite material, particularly in the area surrounding the passenger entry/exit doors. When a boarding ramp comes into contact with such an area, the impact of the ramp against the fuselage may cause tiny amounts of internal fractures within the material. This is especially true if the boarding ramp was moving too fast upon impact. Though superficial damage to the surface of the material may also result, such visible damage is a poor indication of whether or not sub-surface damage is present, or an amount of such sub-surface damage.
Thus, techniques and devices have been developed to test materials for such sub-surface damage. One such device is described in U.S. Pat. No. 7,222,514 to Kollgaard et al. The Kollgaard device includes a calibration mode and a test mode. During calibration, the device is applied to a surface of an object that is similar in composition to a composite material to be tested. Said object is referred to as the ‘reference sample’. The device transmits an ultrasonic pulse (which may otherwise be referred to herein as a calibration pulse, or merely a pulse) to the reference sample. This causes a return echo pulse to travel through the reference sample back to the device. Such a return echo pulse is a reflection of the originally transmitted ultrasonic pulse due to a mismatch in the acoustic impedance between the rear surface of the reference sample and the adjacent air. The amplitude of the return echo pulse and the time of flight (i.e., the time from when the device transmits the calibration pulse through the front surface of the material to the time when the device receives the return echo) are stored as calibration readings. Other aspects of the return echo pulse, such as signal envelope, may also be stored. Typically, however, only the time of flight is needed.
In test mode, the Kollgaard device is applied to the material to be tested, and an ultrasonic pulse is sent through that material. If a return echo pulse comes back to the device in less time than the calibration readings indicate a pulse should need to reach the back end of the material and return, and perhaps with a change in amplitude, the device informs the user that sub-surface damage may be present in that location. That location is noted and later tested using more advanced methods, so as to determine whether or not the material needs to be repaired or replaced, or if structure containing the material is safe for use. Note that if there is a large amount of subsurface damage within the material, a return echo pulse may not be detected due to the damage preventing a sufficient amount of pulse energy from reaching the device. Alternatively, if a return echo pulse comes back to the device having the same or similar amplitude and time of flight as the calibration pulse, then sub-surface damage is likely not present in the material.
The Kollgaard device informs a user of possible sub-surface damage through use of one or more light-emitting diodes (LEDs). If sub-surface damage is detected, one LED is illuminated; if there is no sub-surface damage, another LED is illuminated. Further, other information, such as the depth of damage, and whether the device is calibrated or not, is also shown through LED illumination. The Kollgaard device does not provide any other data, or more specific data, relating to possible damage present in the material.